A surface-active agent or surfactant is a substance that lowers the surface tension of the medium in which it is dissolved, and/or the interfacial tension with other phases. Accordingly, it is positively adsorbed at the liquid/vapour and/or at other interfaces.
Surface-active agents are widely used industry, for instance in foods, cleaning compositions and personal care products. In foods, they are used to achieve emulsions of oily and water-phases, such as in fat spreads or mayonnaise. In laundry cleaning applications, they are used to solubilise dirt and keep it is solution, so that it can be effectively removed from the fabric.
For cleaning applications, the surface-active compounds may be chosen from anionic, cationic, nonionic, amphoteric and zwitterionic surfactants. Many suitable surface-active compounds are available and are fully described in the literature, for example, in “Surface-Active Agents and Detergents”, Volumes I and II, by Schwartz, Perry and Berch.
The most commonly used detergent-active compounds are soaps and synthetic non-soap anionic and nonionic compounds. Examples of anionic surfactants include alkylbenzene sulphonates, particularly linear alkylbenzene sulphonates having an alkyl chain length of C8-C15; primary and secondary alkylsulphates, particularly C8-C15 primary alkyl sulphates; alkyl ether sulphates; olefin sulphonates; alkyl xylene sulphonates; dialkyl sulpho-succinates; and fatty acid ester sulphonates. Sodium salts are generally preferred.
Examples of nonionic surfactants include the primary and secondary alcohol ethoxylates, especially the C8-C20 aliphatic alcohols ethoxylated with an average of from 1 to 20 moles of ethylene oxide per mole of alcohol, and more especially the C10-C15 primary and secondary aliphatic alcohols ethoxylated with an average of from 1 to 10, preferably 3 to 7 moles of ethylene oxide per mole of alcohol. Non-ethoxylated nonionic surfactants include alkylpolyglycosides, glycerol monoethers, and polyhydroxy-amides (glucamide).
The choice of the surface-active material (surfactant), and the amount present, will depend on the intended use of the detergent composition. For fabric washing compositions, different surfactant systems may be chosen, as is well known to the skilled formulator, than for handwashing products or mechanical dishwashing products.
In foods, surface-active materials are commonly used to prepare emulsions. Edible emulsions are used as a base for many types of food products. Mayonnaise compositions, for example, comprise edible oil-in-water emulsions that typically contain between 80 to 85% by weight oil, and egg yolk, salt, vinegar and water. Mayonnaise compositions are enjoyed by many consumers, and particularly, on sandwiches, in dips, with fish and other food applications. The oil present in the edible emulsions used in such food products is generally present as droplets dispersed in the water phase. In addition to droplet size and the amount of droplets dispersed, the close packing of the oil droplets results in the characteristic Theological behaviour of the emulsions used to make the desired food product, such as mayonnaise or margarine.
The surface-active agents that are most commonly used in food applications comprise low molecular weight emulsifiers that are primarily based on fatty acid derivatives. Examples include: lecithin's, monoglycerides (saturated and unsaturated), polysorbate esters (Tweens), sorbitan esters (Spans), polyglycerol esters, propylene glycol monostearate, sodium and calcium stearoyl lactylates, sucrose esters, organic acid (lactic, acetic, tartaric, succinic) esters of monoglycerides. Proteins and other surface-active biopolymers can also be used for this purpose. Typical examples of food proteins include milk proteins (caseins and whey proteins), soy protein, egg protein, lupin protein, pea protein, wheat protein. Examples of other surface-active biopolymers include gum Arabic, modified surface active pectin and OSA modified starch.
Recently, the interest in the study of solid particles as emulsifiers of dispersed systems has been re-awakened. Much of this activity has been stimulated by the research of Binks and co-workers (Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21), though the principles of such stabilisation were observed at least 100 years ago (Ramsden, W. Proc. R. Soc. London 1903, 72, 156). The advantage of particle stabilisation is that it is almost impossible to displace an adsorbed particle once adsorbed to an interface. This gives particle stabilised emulsions and foams excellent stability, especially with respect to ripening mechanisms such as disproportionation.
Whilst the use of particles to stabilise o/w, w/o and duplex emulsions and foams has been amply demonstrated in recent years, much less research has been carried out on non-spherical structures with respect to the stabilisation of interfaces. Furthermore, it has recently been demonstrated by Alargova et. al. (Langmuir, 2004, 20, 10371), that epoxy rods can be used to provide interfacial stabilisation to emulsions and foams.
Notwithstanding the fact that many surface-active materials are known and available, there is a continuous need for new alternative or improved surface-active materials, especially environmentally friendly surface-active materials having good biodegradability properties. It is therefore an object of the present invention to provide such surface-active materials. It is a further object to provide surface-active materials that are useful in the stabilisation of emulsions and foams.
Surprisingly, it has now been found that one or more of the above-mentioned objects can be achieved by the surface-active material according to the invention, which is characterised in that it comprises fibres which have been modified so as to impart surface-active properties onto said fibres and giving it a contact angle between 60° and 120°.
The present inventors have found that the shape and size are of critical importance for the colloidal stability of foams and emulsions. Rod like (fibril) shapes are much more efficient then spherical particles. Another key factor for good foam and emulsion stabilisation is particle contact angle at oil/water or air/water interface, which must be as close to 90° as possible. The rod-like structures must therefore be amphiphathic in design (o/w and w/o stabilisation depends on the relative balance between hydrophobicity and hydrophilicity).
Rod- and fibre-like structures such as microcrystalline cellulose (MCC) and natural plant fibres have been used in food systems. Such structures often find application as bulk structuring agents, providing a contribution to the rheology of a formulation, without showing a tendency to adsorb at interfaces or exhibiting surface-active properties.